1. Field of the Invention
This invention relates to a method for compressing a shaped body of compacted powder, especially of a refractory material, into a more compact body that is substantially free of voids and closely approaches theoretical density.
2. Description of the Prior Art
Techniques have been developed for the isostatic compaction of objects prepared from powders of various materials including metals and ceramics. These techniques have varied with the characteristics of the material under consideration as well as other factors. Isostatic compaction is the application of pressure uniformly on all sides of an object to obtain uniformly dense components. For example, isostatic compaction has been useful in the fabrication of many types of components in powder metallurgy. Ordinarily, for cold compaction, the powders to be compacted have been placed in a flexible container which is immersed in a fluid and then pressure is applied to the fluid to achieve isostatic densification of the powder body.
More recent techniques of isostatic hot-pressing have involved placing a powdered metal to be densified in a deformable metal jacket. The cladded metal powder is then hot-pressed by means of hot, -pressure gases. These presses require large high pressure cylinders and take considerable time to open, close and to operate.
The primary disadvantage of prior techniques, using piston type compaction of powders, has been their limitation to the production of compacted powdered articles of simple constant or symmetrical cross-sectional area, such as spheres, rods, discs, and plates.
Associated with the foregoing has been the difficult problem of producing high density forms of ceramic materials of a desired complex shape. The superiority of densified ceramic materials is so significant that if such materials were available, they could be used for many applications where other materials such as metals have proven unsatisfactory or have a short life. Accordingly, there has been a need for a method of hot-pressing refractory powdered materials into bodies of complex configuration and with a high density. For example, the ceramic material, silicon nitride (Si.sub.3 N.sub.4), if suitably fabricated, is useful for high temperature gasturbine blades. Also, if suitable processes were available, other materials, such as silicon carbide (SiC), as well as many powdered refractory metals, such as tungsten, could be processed from a conventional powder compact into high density, void-free forms with complex or asymmetric configurations at reasonable cost.
The term "complex shape" as applied to a shaped body being processed in accordance with this invention, denotes a body whose cross-sections vary markedly and/or irregularly as successive horizontal planes are passed through the body in a direction perpendicular to the application of the unidirectional pressure in a press carrying out the method of this invention. Usually, such complex shaped bodies will be longer in one dimension than in any other, and the bodies will be placed in the confined pressure-transmitting powder in a direction parallel to the application of the unidirectional pressure. It will be understood that, while the method of this invention gives unexpected and outstanding results with complex shaped bodies, the invention obviously can be applied with good results to simple or symmetrically shaped bodies.
A publication entitled "Isostatic Hot Pressing" by R. P. Levey (dated June 9, 1965, as a result of work done at Union Carbide Corporation under Contract W-7405-eng-26 with the U.S. Atomic Energy Commission) discloses briefly on page 8 thereof the following hot-pressing process:
"One method of accomplishing the hot press in solid media has been developed at Y-12. The operation employs conventional graphite dies filled with graphite powder. The part to be pressed is buried in the powder and pressed. Since the specimen pressure is applied through a powder medium, it is more uniform than if directly applied from a piston. The efficiency of pressure generation in the radial direction has not been studied. The ceiling pressure available is about 3000 psi."
It will be apparent that this work comprises only a few tests at most or only a single experiment on some undesignated part, with no indication of how the pressing and heating was carried out. No statement of the nature of the end result or what benefit, if any, was secured is set forth.
Compaction of an object of a powdered material by applying pressure unidirectionally to the body has been limited to compaction in rigid metal dies. The final product is an axially deformed member.